As it is well known, the trend in the semiconductor world is that as electronic devices become smaller and faster, they inevitably become more sensitive to electro static discharge (ESD).
In particular, typically the most ESD sensitive devices are class zero devices, which can resist ESD events of less than 250 Volts (V). After assembly, a class zero device goes through many handling stages and each stage comes with its own threats of ESD event occurrence. For example, if the electric contacts of a charged device touch another material or object that is not ESD safe, at any stage from assembly stage to testing and packing stage, there is a good possibility for the device to be damaged by an ESD event. Likewise, if an uncharged device touches a surface or object which is not sufficiently grounded, an ESD event and resulting damage is probable.
It is also well known that the Ball Grid Array (BGA) is a kind of package which is very prone to ESD events. FIG. 1A shows a BGA package 1 comprising a substrate 2 including a plurality N of electric contacts realized on its surface and comprising copper pads 3a connected to solder balls 3b. As it appears from the top view of the BGA package/shown in FIG. 1B, the solder balls are completely exposed, causing the package to be susceptible to ESD events and physical damage. In fact, a substantial percentage of the final yield loss in BGA is due to physical defects on the balls such as squashed, dented, sheared, and missing balls. As an example, the finishing stage alone results in an average yield loss of 0.25% on otherwise good parts.
In addition, after assembly, the ESD sensitive BGA package goes through many handling stages, and each stage comes with its own threats of ESD event occurrence.
The ESD effect is also present in a Quad Flat No-Lead (QFN) package. As shown in FIG. 2A, a QFN package 11 comprises a substrate 12 having on its back surface a plurality N of electric contacts comprising contact pads 13 without solder balls. So, the contacts are typically less prone than the contacts in a BGA package to physical damage. Nevertheless, the pads of the QFN package are still exposed, and thus the QFN package is typically as susceptible to ESD damage as the BGA package.
Many techniques to reduce the impact of ESD on devices have been implemented. At present, a lot of investment, resources, and time are utilized to avoid the occurrence of such damages. For example it is recommended that new, expensive ESD-safe floor mats be installed everywhere a device will be handled, ionizers should be installed in the environment where the devices are handled, and all personnel in charge of the manufacturing of the device should wear heel straps and wrist straps. Furthermore, trays and all material which could come in contact with the devices, and even material which could not come in contact but could be a short distance away (and thus be capacitively coupled to a device), should be ESD safe and properly grounded.
Nevertheless there are still some problems in implementing such safe measures due to the fact that ESD safe material is very expensive, and requires regularly scheduled checking and maintenance. Some ESD dissipative materials also have a finite lifetime and must be changed periodically.
ESD damage affects production yields, product quality, reliability and profitability, and manufacturing costs. It has been estimated in an article: “The ESD Control Process is a Tool for Managing Quality”, Electronic Packaging and Production, April 1990, pp 50-53, which is incorporated by reference, that the actual cost of ESD damage to the electronic industry runs into billions of dollars annually when including the associated costs of repair and rework, shipping, labor, and overhead.
Several solutions are known in the prior art to reduce the ESD events, but they are all based on modifying and monitoring the device's environment during all manufacturing processes. These solutions however, may be relatively complex, unreliable, and unmanageable, and the devices for which these solutions are applied may still be prone to ESD damage.